The present disclosure relates to a technology for transferring, onto a sheet, a toner image carried on an image carrier (for instance, on a drum surface of a photosensitive drum), with use of a transfer member (for instance, a transfer roller) to which a transfer bias voltage is applied.
Image formation by an electrophotographic system includes a charging step, an exposure step, a developing step, a transferring step, and a fixing step.
In the transferring step, a transfer bias voltage is applied to a transfer roller (example of a transfer member). The transfer bias voltage is a bias voltage for use in transferring toner for forming a toner image onto a sheet. By performing the above operation, a toner image carried on the drum surface is transferred onto a sheet when the sheet is passed through a nip portion formed between the drum surface and the transfer roller.
The electric potential on the drum surface of the photosensitive drum may fluctuate by application of transfer bias voltage. Regarding the above drawback, there is described the following case, in which the polarity of electric charge for charging the drum surface in a charging step is positive, and the polarity of transfer bias voltage is negative. In this case, upon application of transfer bias voltage of negative polarity to the drum surface by a transfer roller, a region charged with negative electric charge may be formed on the drum surface. In the case where the drum surface has such a physical property that the drum surface is sensitive only to positive polarity in removing static electricity from the drum surface, it is impossible to remove the negative electric charge from the drum surface. As a result, the electric potential on the drum surface may fluctuate. The electric potential fluctuation on the drum surface may cause density variation in an image transferred onto a sheet.
The electric potential fluctuation on the drum surface may also occur because of the following reason. Since a transfer bias voltage is applied to the transfer roller, electric current flows via the nip portion between the drum surface and the transfer roller. If there is a sheet in the nip portion, the electrical resistance of the nip portion is large, as compared with a case, in which there is no sheet in the nip portion. As a result, the electrical resistance of the nip portion is sharply changed when the leading end or the trailing end of a sheet is passed through the nip portion. This causes an excessive current flow between the drum surface and the transfer roller. Consequently, the electric potential on the drum surface varies, and electric potential fluctuation may occur on the drum surface in the form of streaks, for instance. The electric potential fluctuation in the form of streaks appears as streak-like noise on an image transferred onto the sheet.
There has been proposed a technique for eliminating such electric potential fluctuation on the drum surface in the form of streaks. In the above technique, there is provided current detecting means for detecting electric current flowing from a transfer roller to a drum surface in applying a transfer bias voltage to the transfer roller. In the case where the electric current detected by the current detecting means has exceeded a predetermined threshold value when the trailing end of a preceding sheet is passed through the nip portion, the number of times of charging the region on the drum surface where the electric potential has fluctuated in the charging step is increased by delaying the timing of feeding a next sheet. In this way, the electric potential fluctuation is eliminated.
An object of the present disclosure is to provide an image forming apparatus that enables to prevent electric potential fluctuation on an image carrier.